Battery-driven devices, such as mobile phone terminals, smart phones, tablet terminals, notebook computers, portable audio players, and the like, incorporate a rechargeable secondary battery and a charging circuit for charging the battery. Some charging circuits charge secondary battery using a DC voltage (bus voltage VBUS) supplied from the outside via a USB cable or a DC voltage from an external AC adapter.
Currently, charging circuits installed in mobile devices are mainly based on a standard called USB Battery Charging Specification (hereinafter referred to as BC standard). There are several types of USB hosts or chargers (hereinafter collectively referred to as USB power feed devices). The BC revision 1.2 standard defines SDP (Standard Downstream Port), DCP (Dedicated Charging Port) and CDP (Charging Downstream Port) as types of USB power feed devices. In addition, a current (current capacity) which can be supplied by a USB power feed device is defined in the BC revision 1.2 standard according the type of USB device. Specifically, 1500 mA is defined for DCP and CDP and 100 mA, 500 mA and 900 mA are defined for SDP depending on the version of USB.
A standard called USB Power Delivery (hereinafter referred to as PD standard) has been formulated as the next generation secondary battery charging format and system using USB. According to the PD standard, suppliable power increases greatly from 7.5 W of the BC standard to the maximum of 100 W. Specifically, the PD standard allows a supply of voltage (specifically, 12 V or 20 V) higher than 5 V as a USB bus voltage, and thus a supply of charging current (specifically, 2 A, 3 A or 5 A) larger than that in the BC standard is also allowed. The USB PD is also supported in the USB type-C standard.
FIG. 1 is a block diagram of a power feed system 100R studied by the present inventor. This power feed system 100R conforms to the USB type-C standard and includes a power feed device 200R and a power receiving device 300R which are interconnected via a USB cable 102. The power receiving device 300R is mounted on a battery-driven electronic apparatus 400R such as a smart phone, a tablet terminal, a digital camera, a digital video camera, a portable audio player or the like.
The power feed device 200R includes a power supply circuit 202, a PD controller of a power feed side (hereinafter referred to as a power feed side controller) 204, and a receptacle 206. The USB cable 102 is detachably connected to the receptacle 206. There is also a charging adapter in which the USB cable 102 is integrated with the power supply device 200R without the receptacle 206.
The receptacle 206 includes a VBUS terminal for supplying a bus voltage VBUS, a GND terminal for supplying a ground voltage VGND, and a CC (Configuration Channel) terminal. The power supply circuit 202 generates the bus voltage VBUS. The power supply circuit 202 may include an AC/DC converter which receives an AC voltage of 100 V from an external power supply (for example, a commercial AC power supply) (not shown) and converts it to a DC bus voltage VBUS. The bus voltage VBUS generated by the power supply circuit 202 is supplied to the power receiving device 300R via a bus line of the USB cable 102.
The power feed side controller 204 is connected to a PD controller of the power receiving side (hereinafter referred to as a power receiving side controller) 306 via the USB cable 102. The power feed side controller 204 and the power receiving side controller 306 provide a communication function between the power feed device 200R and the power receiving device 300R. In the USB PD standard, the CC terminal is omitted and it is configured to communicate between the power feed device 200R and the power receiving device 300R via a bus line.
The electronic apparatus 400R includes a receptacle 402 and a load 404 in addition to the power receiving device 300R. The load 404 includes a CPU, a memory, a liquid crystal display, an audio circuit and the like. The power feed device 200R is detachably connected to the receptacle 402 via the USB cable 102.
The power receiving device 300R includes a battery 302, a charging circuit 304, the power receiving side controller 306, a microcontroller 308 and a power supply circuit 320. The battery 302 is a rechargeable secondary battery. The charging circuit 304 receives the bus voltage VBUS (also referred to as an adapter voltage VADP on the side of the power receiving device 300R) from the power feed device 200R via the USB cable 102 and charges the battery 302 with the bus voltage VBUS. The charging circuit 304 is composed of a step-down DC/DC converter, a linear regulator or a combination thereof and, therefore, must satisfy a relationship of VADP>VBAT in order to charge the battery 302.
A system voltage VSYS corresponding to at least one of the bus voltage VBUS (adapter voltage VADP) and the voltage VBAT of the battery 302 is supplied from the charging circuit 304 to the power supply circuit 320. The power supply circuit 320 steps up or steps down the system voltage VSYS and supplies a power source voltage VDD to each circuit block of the load 404. The power supply circuit 320 is a multi-channel power supply including a power management IC (Integrated Circuit), a DC/DC converter, a linear regulator and the like.
The power receiving side controller 306 is a port controller related to the USB type-C and communicates (negotiates) with the power feed side controller 204 via the CC line. The microcontroller 308 generally controls functions related to USB charging and power management in the power receiving device 300R.
FIG. 2 is a graphical diagram showing a process of determining the bus voltage VBUS in the USB PD standard of FIG. 1. Here, as an example, it is assumed that the power feed device 200R supports 5 V, 9 V and 12 V. The battery 302 is, for example, a two-cell lithium ion battery and the battery voltage VBAT varies in a range of 6 V to 9 V. In order to charge this battery 302, a bus voltage VBUS of 9 V or higher is required. A priority of the bus voltage VBUS (for example, 9 V, 12 V, 20V) requested to the power feed device 200R is defined in the power receiving side controller 306 or the microcontroller 308.
When the power feed device 200R is connected to the power receiving device 300R via the USB cable 102 at time t0, an initial bus voltage VBUS of 5 V is supplied to the power receiving device 300R, thereby establishing a link between the power receiving side controller 306 and the power feed side controller 204 (t1). The power receiving side controller 306 communicates (negotiates) with the power feed side controller 204 and requests the power feed side controller 204 to provide the bus voltage VBUS of 9 V or higher (t1 to t2). In this example, since the power feed device 200R supports 9 V, the request of the bus voltage VBUS=9 V is accepted. Then, after time t2, the bus voltage VBUS of 9 V is supplied and a charging operation starts. The battery voltage VBAT rises with the charging operation.
The present inventor has studied the power feed system 100R of FIG. 1 and has recognized the following problems.
The efficiency of the charging circuit 304 depends on its input voltage VADP and the battery voltage VBAT (the system voltage VSYS). In the conventional power feed system 100R, once the bus voltage VBUS is set by negotiation, the set value of the bus voltage VBUS is thereafter fixed. That is, the input voltage of the charging circuit 304 is constant. On the other hand, the battery voltage VBAT varies depending on the SOC (State Of Charge) of the battery 302. Therefore, in the process of charging (or discharging) the battery 302, the efficiency of the charging circuit 304 may be significantly lowered depending on the magnitude of the battery voltage VBAT. Reduction in the efficiency of the charging circuit 304 is undesirable from the viewpoint of energy saving and also causes a problem of prolonging the charging time of the battery 302.
In addition, when the system is operated with the bus voltage VBUS constant, if the power consumption of the power receiving device 300R exceeds a power which can be supplied by the power feed device 2008, a situation may occur in which the bus voltage VBUS drops to stop the charging.
The above problems may occur not only in the USB PD standard or the USB type-C standard but also in a power feed system having a similar configuration and protocol.